1. Field of the invention
The present invention is related generally to correlative systems and more particularly, to a system and circuitry for indicating the state of the system; i.e., whether it is locked onto a periodic signal or not.
2. Description of the prior art
The prior art shows correlative tracking systems such as phase-locked-loops, wherein a quadrature correlation signal is used for lock indication by comparing it with a threshold value. In this device the definition of the locked state depends on the cut-off frequency, the characteristics of the low-pass filter, the lock detector circuit, on the amplitude of the input voltage, on the selected threshold, on the difference of the input frequency and the oscillator frequency, and on the phase relation of the two voltages. It is difficult to select the cut-off frequency of the low-pass filter and the threshold value so that indication of the locked state following acquisition and leaving the locked state is rapid and certain.
The U.S. Pat. No. 3,893,042 describes a lock indicator for phase-locked loops utilizing a second auxiliary phase-locked loop having a slightly different basic frequency to which the incoming signal is fed in parallel with the primary phase-locked loop. If the signal is acquired by both, an exclusive OR-gate produces a continuous zero output, indicating that a lock-up has occurred. For high background noise, no lock indication is given, even if the primary loop is still locked.